1. Field of the Invention
This invention relates to a composite substrate having a dielectric and an electrode, a method for preparing the same, and an electroluminescent (EL) device using the same.
2. Background Art
The phenomenon that a material emits light upon application of an electric field is known as electroluminescence (EL). Devices utilizing this phenomenon are on commercial use as backlight in liquid crystal displays (LCD) and watches.
The EL devices include dispersion type devices of the structure that a dispersion of a powder phosphor in an organic material or enamel is sandwiched between electrodes, and thin-film type devices in which a thin-film phosphor sandwiched between two electrodes and two insulating thin films is formed on an electrically insulating substrate. For each type, the drive modes include DC voltage drive mode and AC voltage drive mode. The dispersion type EL devices are known from the past and have the advantage of easy manufacture, but their use is limited because of a low luminance and a short lifetime. On the other hand, the thin-film type EL devices have markedly spread the practical range of EL device application by virtue of a high luminance and a long lifetime.
In prior art thin-film type EL devices, the predominant structure is such that blue sheet glass customarily used in liquid crystal displays and plasma display panels (PDP) is employed as the substrate, a transparent electrode of ITO or the like is used as the electrode in contact with the substrate, and the phosphor emits light which exits from the substrate side. Among phosphor materials, Mn-doped ZnS which emits yellowish orange light has been often used from the standpoints of ease of deposition and light emitting characteristics. The use of phosphor materials which emit light in the primaries of red, green and blue is essential to manufacture color displays. Engineers continued research on candidate phosphor materials such as Ce-doped SrS and Tm-doped ZnS for blue light emission, Sm-doped ZnS and Eu-doped CaS for red light emission, and Tb-doped ZnS and Ce-doped CaS for green light emission. However, problems of emission luminance, luminous efficiency and color purity remain outstanding until now, and none of these materials have reached the practical level.
High-temperature film deposition and high-temperature heat treatment following deposition are known to be promising as means for solving these problems. When such a process is employed, use of blue sheet glass as the substrate is unacceptable from the standpoint of heat resistance. Quartz substrates having heat resistance are under consideration, but not adequate in such applications requiring a large surface area as in displays because the quartz substrates are very expensive.
It was recently reported that a device was developed using an electrically insulating ceramic substrate as the substrate and a thick-film dielectric instead of a thin-film insulator under the phosphor, as disclosed in JP-A 7-50197 and JP-B 7-44072.
FIG. 2 illustrates the basic structure of this device. The EL device in FIG. 2 is structured such that a lower electrode 12, a thick-film dielectric layer 13, a light emitting layer 14, a thin-film insulator layer 15 and an upper electrode 16 are successively formed on a substrate 11 of ceramic or similar material. Since the light emitted by the phosphor exits from the upper side of the EL structure opposite to the substrate as opposed to the prior art structure, two electrodes are provided on upper and lower sides of the EL structure.
In this device, the thick-film dielectric has a thickness of several tens of microns which is about several ten to several thousand times the thickness of the thin-film insulator. This offers advantages including a minimized chance of breakdown caused by pinholes or the like, high reliability, and high manufacturing yields.
Use of the thick dielectric causes a voltage drop across the phosphor layer which is overcome by using a high-permittivity material as the dielectric layer. Use of the ceramic substrate and the thick-film dielectric permits a higher temperature for heat treatment. As a result, it becomes possible to deposit a light emitting material having high light-emitting characteristics, which was impossible in the prior art because of inclusion of crystal defects.
However, the light emitting layer formed on the thick-film dielectric layer has a thickness of several hundreds of nanometers which is about {fraction (1/100)} of that of the thick-film dielectric layer. This requires that the surface of the thick-film dielectric layer be smooth to a level below the thickness of the light emitting layer although a conventional thick-film process is difficult to form a dielectric layer having a fully smooth surface.
If the surface of the dielectric layer is not smooth, it is impossible to uniformly form a light emitting layer thereon, and/or a delamination phenomenon occurs between the dielectric layer and the light emitting layer, which can cause a substantial degradation of display quality. Therefore, the prior art technology requires smoothing operations of removing large asperities by polishing and removing fine asperities by a sol-gel process.
However, it is technically difficult to polish large surface area substrates for display and other applications. The sol-gel process cannot accommodate for large asperities when used alone. Additionally, an increased cost of stock material and an increased number of steps involved are undesirable.
An object of the invention is to provide a method for preparing a composite substrate which has minimized surface asperities on a dielectric layer, which are otherwise developed under the influence of an electrode layer and a ceramic substrate, which eliminates a need for polishing step, which is easy to manufacture, and which is applicable to the fabrication of a thin-film light-emitting device of high display quality, as well as the resulting composite substrate and a thin-film EL device using the same.
The above object is attained by the present invention as constructed below.
(1) A method for preparing a composite substrate, comprising the steps of:
forming at least an electrode and a green dielectric layer according to a thick-film technique on an electrically insulating substrate, thereby providing a composite substrate precursor,
smoothing the surface of the precursor by WIP process, and
firing to complete the composite substrate.
(2) The method of (1) wherein the WIP process is effected at a temperature which is not lower than 40xc2x0 C. or the glass transition temperature (Tg) of a binder in said green dielectric layer.
(3) The method of (1) wherein said green dielectric layer uses a thermoplastic resin as a binder.
(4) The method of (1) wherein during the heat compression step, a vacuum package is used to avoid contact of the composite substrate precursor with a pressure transmitting fluid, and a resin film is interposed between the vacuum package and the green dielectric layer.
(5) The method of (4) wherein a parting agent is disposed below the resin film.
(6) A composite substrate prepared by the method of (1), a functional thin film being to be formed on the resulting thick-film dielectric layer.
(7) An EL device comprising at least a light emitting layer and a transparent electrode on the composite substrate of (6).